1. Field of the Invention
The present invention relates to an electro-optical device using a pixel electrode and nonlinear resistance elements for a unit pixel formed along an operating electrode.
2. Description of the Related Background Art
A liquid crystal display panel has advantageous features such as a thin thickness and lightweight display panel having low power consumption and is currently used in a variety of applications, for example, in laptop or book type personal computers. Of conventional liquid crystal display panels, an active matrix display panel has received a great deal of attention due to its high resolution, and high image quality. Typical active elements are a three-terminal element using a thin film transistor, and a two-terminal element represented by a nonlinear resistance element (e.g., a MIM (Metal Insulator Metal) resistance element) and a p-n junction thin film diode.
Of these conventional active elements, a three-terminal element is produced through a complicated fabrication process, so that a low yield and high cost are inevitable in inconvenience. The diode referred above has a low breakdown voltage and is susceptible to static electricity. To the contrary, a nonlinear resistance element has a simple structure and a high breakdown voltage of 25 V or more. Therefore, the nonlinear resistance element can be utilized for a display panel having a large area at low cost.
FIG. 2A is a circuit diagram showing an X-Y matrix panel circuit in a conventional electro-optical device using nonlinear resistance elements, FIG. 2B is a sectional view showing a structure of the unit cell of the device, and FIG. 2C is a plan view showing a structure of the nonlinear resistance element. Referring to FIGS. 2A to 2C, 100 to 1,000 row electrodes 1 and 100 to 1,000 column electrodes 2 are generally formed on a substrate B and a counter substrate A, respectively. Each X-Y intersection has a pixel electrode 22 and a nonlinear resistance layer 21 to constitute a nonlinear resistance element 4 connected to a corresponding one of the column electrodes 2. An electro-optical material 3 is sealed between the substrates A and B. When this structure is used as a liquid crystal display panel, the substrates A and B are generally made of glass, the row and pixel electrodes 1 and 22 are generally made of ITO (Indium Tin Oxide), the column electrode 2 is generally made of Cr or Al, and the nonlinear resistance layer 21 is generally made of Si-rich silicon nitride.
The operation of the liquid crystal display panel of this type is performed as follows. The row electrodes 1 (column electrodes 2) in FIGS. 2A to 2B are sequentially selected one by one, and data is written by each corresponding column electrode 2 (row electrode 1) in the selected period. At this time, in order to perform a display with a sufficient contrast ratio (e.g., a contrast ratio of 10:1, or more), an RMS (Root-Mean-Square) voltage applied to a liquid crystal at the selected pixel must be higher than a saturation voltage of the liquid crystal, and an RMS voltage applied to the liquid crystal during a non-selected pixel must be lower than a threshold voltage of the liquid crystal. The nonlinear resistance element 4 has characteristics defined such that a resistance of the element is exponentially changed by the voltage applied thereto. For this reason, a higher voltage is set to be applied to the selected pixel of the nonlinear resistance element 4 within a selected period to decrease the element resistance (e.g., 10.sup.8 .OMEGA. or less), so that charges can be easily injected into the pixel electrode 22. At the half-selected pixel, a voltage applied to the nonlinear resistance element 4 is suppressed to be low within the selected period. In this case, the resistance of the element is not decreased (e.g., 10.sup.9 .OMEGA. or more), and charges tend not to be injected into the pixel electrode 22. During a retention period, a low voltage is applied to both the selected and the non-selected pixels of the nonlinear resistance element 4, and the resistance of the non-linear element is kept high (e.g., 10.sup.11 .OMEGA. or more), thereby increasing the charge retention capacity of the element. As is apparent from the above description, when the nonlinear resistance element is used, the RMS voltage applied to the liquid crystal can be kept higher than the saturation voltage of the liquid crystal at the selected pixel and lower than the threshold voltage at the non-selected pixel. Therefore, a high contrast ratio can be obtained even if the number of dots is increased.
In order to perform a display on this liquid crystal display panel, it is important to determine the driving voltage, the composition and thickness of the nonlinear resistance layer, and the structure of the nonlinear resistance element 4 so as to obtain desired resistances of the nonlinear resistance element 4 during the selected and retention periods. It is also important to increase a ratio of a capacitance C.sub.LC of a liquid crystal portion of each pixel to a capacitance C.sub.1 of a nonlinear resistance element portion of each pixel (at least C.sub.LC /C.sub.I .gtoreq.5) so as to obtain a sufficient operating margin and to compensate for a distribution of element characteristics and their deviation over time.
As described above, although the display panel using nonlinear resistance elements can have a large capacity, a problem is posed if a gray-scale display is to be performed in addition to simple turned on/off display, as compared with a three-terminal element due to the following reason. Since each three-terminal element is operated as a perfectly independent switch, a given charge injected into one pixel rarely receives an influence of a charge written in another pixel during a retention period of the given charge. In the nonlinear resistance element, however, a very small current (up to about 10 pA) flows even during the retention period. Data stored in other pixels gradually influence data stored in respective pixels through the corresponding column electrodes (row electrodes). For this reason, the RMS voltage applied to the liquid crystal in accordance with a display pattern is gradually deviated from the predetermined value. In addition, since the resistance of the element greatly influences the charge injection capacity and the charge retention capacity, element characteristics vary within the panel surface and are shifted due to deteriorations over time. At this time, changes in element characteristics cause a direct change in the RMS voltage applied to the liquid crystal. For this reason, when an RMS voltage applied to the liquid crystal is to be controlled with high precision as in a multilevel gray-scale display, a contrast difference is caused to make it difficult to perform a normal display. This difference is increased when the panel size is increased and the number of dots is increased, resulting in inconvenience.